An instrument in the sense of a traditional mechanical music instrument is 'a device created or adapted for the purpose of making musical sounds.' [w1] To produce these sounds the player touches and in many cases also holds the instrument with his body. By doing so mechanical force originating from the player's body is injected into the composited organic and mechanical system of the player's and the instrument's bodies.

The sounding acoustic vibrations result from the intricate interactions between the two bodies entangled in an emergent complex system. Emergent understood as 'complex systems and patterns aris[ing] out of a multiplicity of relatively simple interactions.' [w2] These are the multitudinous interactions between the player's skin, bone structure, muscle tension, etc and the instrument's string elasticity, body material, resonating chamber dimensions, etc. There is not one distinct linear 'signal path' from muscle contraction to acoustic wave but rather a multitude of parallel, interfering and resonating vibrations propagating through the composite space of the two entangled bodies.

As such, 'at the mechanical level, the player is an integral part of this process. It is impossible to dissociate a flute player's touch and breath from a sounding flute.' [Norman, Waisvisz, Ryan] Two perspectives derive from that:
1. An untouched instrument is incomplete. It is completed by the human body holding it. As such the instrument is the composite of both bodies and doesn't exist without the human.
2. Instruments are extensions of the human body, a 'type of prosthesis.' [Raes]
From here on instrument is understood as the composite of the mechanical and human bodies. (Please note that an instrument in this sense can exist without a mechanical body: a singer's voice for example.)

As stated, the instrument's composite body defines the space from which the acoustic waves emanate. Lets call this the instrument's acoustic space. Some dimensions of a guitar's acoustic space are the number, length, thickness and tension of strings. The player's skin and bone structure are as well. The actual physical space/room/environment the playing takes place in is too. Its acoustics shape the sounding result and can be played as well.

The range of interaction possibilities with the instrument's acoustic space is the player's expression space (or expression field [Waisvisz]). Fret position of a finger, plucking technique, force and speed are traditional dimensions of a guitar's expression space. More adventurous ones like knocking the body and scratching the neck are also part of it.

Although the acoustic space is theoretically quantifiable in a generic model the expression space isn't. Expression space is individual to the player in question.

On playing:

The mechanical aspect of playing an instrument is manipulating its acoustic space, ie. causing vibrations and modulating their propagation. A performer does this by actions or gestures in expression space. Through gestures he maps the dynamics of the acoustic space's many dimensions in time. In most cases it is impossible for a performer to control each of the many dimensions individually. Luckily, one gesture can operate in several dimensions at once. In fact, classical mechanical instruments have been developed over time to bind as many dimensions as possible to specific gestures so that they will produce specific tones clearly and consistently.

This is not to say that playing the instrument is reduced to pressing a single button so the virtuosic piece automatically rolls out of it. (Such a device reduces the human component to nearly zero and does not qualify as an instrument. A record playback device is more likely.) A vast portion of musical expressivity is a result from the physical effort involved in performing it. Joel Ryan: 'It is the element of energy and desire, of attraction and repulsion in the movement of music. But effort is just as important in the formal construction of music as for its expression: effort maps complex territories onto the simple grid of pitch and harmony.' [Ryan]

During play a state can emerge 'where a fast closed loop establishes itself between the musical intention, the muscular effort and actions, the mechanical response and the sonic feed back and the perception of this whole loop. This happens so fast that one seems to act immediately in sound and not in 'terms of sound' and not in terms of 'control'. Composition [and] performance melt into a single state of emerging - timbral - expression.' [Waisvisz] This the ideal state of embodied musical performance.

Embodiment occurs not only on the performer's side but on the audience's as well. It happens on two levels. The first is the bodily 'feeling' of the measure (dynamics) of music when listening to it. [Ryan] It is what makes us tap our feet, nod our heads and dance to it. The second is when listening 'becomes a mimetic metaphor in the imagination'. [Raes] Listening induces movements which could hypothetically produce the sounds heard. It's what makes us play air guitar. Both are greatly enhanced when the performance is not only heard but also seen. Seeing the performer(s) at work, making the gestures, putting the effort in, slipping in the fast closed loop state, boosts the audience's embodied experience.

Personal approach:

The notion of the acoustic instrument as a composite mechanical and human body is the main guideline in the current development of my electronic audiovisual instruments. I am seeking for a comparably intricate entanglement of my human body and the electronic system. I seek to make performances a truly embodied experience, for myself as well as the audience. (This of course lies in the field of gestural/tangible electronic instrument development performed at STEIM, Ircam, etc.)

I am developing the instrument such that the performer’s body is an integral part of the generative process. I am trying to find a digital analogue to the integration of human and mechanical bodies in the acoustic domain, at full body scale. Consequently the human body becomes fully immersed in the system’s body, whereas with acoustic instruments the human is merely touching it, not fully inside of it. This full integration ‘extract[s] the interaction from the locked-in, impenetrable virtual space to the human-scale physical space’ [Salter 329] and ties the generative processes to the performer’s full body. In other words, maximum embodiment.